package com.koko.utils;

public class SnowflakeIdGenerator {
    // 开始时间截 (2020-01-01)
    private final long twepoch = 1577836800000L;

    // 机器id所占的位数
    private final long workerIdBits = 5L;

    // 数据标识id所占的位数
    private final long datacenterIdBits = 5L;

    // 支持的最大机器id，结果是31 (这个移位算法可以快速计算出几位二进制数所能表示的最大十进制数)
    private final long maxWorkerId = -1L ^ (-1L << workerIdBits);

    // 支持的最大数据标识id
    private final long maxDatacenterId = -1L ^ (-1L << datacenterIdBits);

    // 序列在id中占的位数
    private final long sequenceBits = 12L;

    // 机器ID向左移12位
    private final long workerIdShift = sequenceBits;

    // 数据标识id向左移17位(12+5)
    private final long datacenterIdShift = sequenceBits + workerIdBits;

    // 时间截向左移22位(5+5+12)
    private final long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits;

    // 生成序列的掩码，这里为4095 (0b111111111111=0xfff=4095)
    private final long sequenceMask = -1L ^ (-1L << sequenceBits);

    private long workerId;
    private long datacenterId;
    private long sequence = 0L; // 序列号
    private long lastTimestamp = -1L; // 上次生成ID的时间截

    public SnowflakeIdGenerator(long workerId, long datacenterId) {
        if (workerId > maxWorkerId || workerId < 0) {
            throw new IllegalArgumentException(String.format("Worker ID can't be greater than %d or less than 0", maxWorkerId));
        }
        if (datacenterId > maxDatacenterId || datacenterId < 0) {
            throw new IllegalArgumentException(String.format("Datacenter ID can't be greater than %d or less than 0", maxDatacenterId));
        }
        this.workerId = workerId;
        this.datacenterId = datacenterId;
    }

    public synchronized long nextId() {
        long timestamp = timeGen();

        if (timestamp < lastTimestamp) {
            throw new RuntimeException(String.format("Clock moved backwards. Refusing to generate id for %d milliseconds", lastTimestamp - timestamp));
        }

        if (lastTimestamp == timestamp) {
            sequence = (sequence + 1) & sequenceMask;
            if (sequence == 0) {
                timestamp = tilNextMillis(lastTimestamp);
            }
        } else {
            sequence = 0L;
        }

        lastTimestamp = timestamp;

        return ((timestamp - twepoch) << timestampLeftShift) |
                (datacenterId << datacenterIdShift) |
                (workerId << workerIdShift) |
                sequence;
    }

    protected long tilNextMillis(long lastTimestamp) {
        long timestamp = timeGen();
        while (timestamp <= lastTimestamp) {
            timestamp = timeGen();
        }
        return timestamp;
    }

    protected long timeGen() {
        return System.currentTimeMillis();
    }
}

